Background and Objective: Relining fiber posts with flowable composites is a procedure used for strengthening the weak walls of endodontically treated teeth. The aim of this study was to evaluate the effect of relining fiber posts with two types of flowable composites on the fracture resistance of the endodontically treated teeth.Materials and Methods: In this laboratory experimental study, 30 intact human maxillary incisors were selected. Root canal therapy was performed for all teeth, and the crown of each tooth was removed up to 2 mm above cementoenamel junction. The teeth were randomly divided into two equal groups of 15 cases. In Group 1, the endodontically treated teeth were restored with fiber post relined with low filler-loaded composite (Alite). On the other hand, in Group 2, the teeth were treated with high filler-loaded composite (Grandino). The relined fiber posts were cemented, and then restored by using cormax II. The fracture resistance of all samples was measured by means of a universal test machine at the crosshead speed of 0.5 mm/min in Newton. The load was applied at an angle of 45˚C to the long axis of the tooth until specimen was fractured. The data were analyzed in SPSS, version 18 using Kolmogorove-Smirnov test and independent t-test at the significance level of 0.05.Results: The mean fracture strengths were 476.47±155.26 and 674.87±155.26 N for groups 1 and 2, respectively. Furthermore, there was a statistically significant difference between the two groups in terms of the fracture strength (P<0.05).Conclusion: According to findings of this study, the application of high filler-loaded flowable composite (Grandino) could significantly increase the fracture resistance of the endodontically treated teeth with weak root as compared to low filler-loaded flowable composite (Alite).